In liquid crystal displays, a light emitted from a cold cathode-ray tube (typically a fluorescent tube) is introduced through a light guiding plate to a display screen. In that case, an optical diffusion film for uniformizing the brightness of the whole screen is used in order to prevent brightness unevenness from occurring. The optical diffusion film is required to have a function of making an image of a light source invisible by diffusing light emitted from the light source and a function of maintaining the brightness of the whole screen without impairing the brightness of the light source. Therefore, the optical diffusion film is required to have properties in which loss by a reflection and absorption is minimized, a scattered light is passed and a parallel light is not passed.
Conventionally, as the optical diffusion film, there has been used a product obtained by matting the surface of a transparent sheet or film such as glass, an acrylic resin, polycarbonate, polyester and the like. In addition, there has been also used a transparent sheet or film to which inorganic particulates such as calcium carbonate, silica particles and the like and transparent resin particles such as polystyrene, PMMA and the like are added. Furthermore, there has been also used a product in which a resin to which the particles are added onto the surface of a transparent sheet and the like is provided as a coating film (for example, Patent Documents 1 to 3).
Incidentally, recently, in liquid crystal displays, a high brightness and thin-profile display is more strongly demanded than before. For this reason, an optical diffusion film in the optical guide system is required to satisfy both a suitable light transmittance and an adequate range of a haze value and have a thin film thickness.
As the optical diffusion film, there is known an optical diffusion material in which a metallic oxide of which particle diameter is longer than the visible light wavelength range is dispersed in a polymer (an organic polymer). For example, in Patent Document 4, there is disclosed an optical diffusion material in which metallic oxide particles having a refractive index different from that of a polymer are dispersed in a polymer. The material may be obtained by a convenient method of mixing the particles with a polymer. However, an optical diffusion material to which metallic oxide particles with almost single size (the particle size distribution is a single dispersion) are added has a problem of exhibiting wavelength dependency that the scattering efficiency of a specific wavelength becomes high and the transmitted light is colored (refer to “Development and Application Techniques of Optical Organic Polymer Materials” p. 183). In addition, there is also a problem that the coagulation of particles is difficult to suppress only by mixing metallic oxide particles with a polymer.
On the other hand, as a method of obtaining a composite by dispersing inorganic compound particles in a polymer while suppressing coagulation, there is known a method of obtaining a polymer-metallic oxide composite with the use of a sol-gel method (refer to Patent Document 5 and Non-Patent Document 1). The sol-gel method is a method in which a metallic oxide may be produced by subjecting a metallic alkoxide, a metallic acetylacetonate, a metallic carboxylate and the like to hydrolysis and polycondensation reaction (sol-gel reaction). The sol-gel method is known as a suitable method for propagating a composite comprising of an inorganic material and an organic material which is decomposed at a higher temperature because a metallic oxide may be synthesized at a relatively lower temperature in a solution. Since a metallic oxide may be synthesized from a reaction at molecular level by the sol-gel method, a metallic oxide with a particle size of a nano level may be dispersed in a polymer. With the use of such characteristics, the sol-gel method is applied especially as a method of producing a nanocomposite of a metallic oxide with a transparent resin material. Such a nanocomposite material is generally known to have increased thermal and mechanical properties without losing transparency, and there are many examples of reports for various resins.
As the examples of using the sol-gel reaction, there are organic and inorganic composite optical films excellent in transparency which are obtained by subjecting a metallic alkoxide compound and the like to sol-gel reaction in a state where a polymer is dissolved in a solvent such as an amide compounds, an urea compounds or the like (Patent Document 6).
In addition, in Patent Document 7, there is disclosed a method in which a metallic alkoxide is impregnated in a polymer so that a concentration gradient may occur, followed by subjecting the metallic alkoxide to polycondensation reaction (sol-gel reaction). The polymer-metallic oxide composite obtained by the method has a structure in which the particle size of the metallic oxide is gradient in the film thickness direction.    Patent Document 1: Japanese Utility Model Examined Application No. Hei 3-26481    Patent Document 2: Japanese Patent Laid-Open Publication No. 2004-198743    Patent Document 3: Japanese Patent Laid-Open Publication No. 2005-55861    Patent Document 4: Japanese Patent Laid-Open Publication No. 2003-109747    Patent Document 5: Japanese Patent Laid-Open Publication No. Hei 8-73739    Patent Document 6: Japanese Patent Laid-Open Publication No. 2005-264038    Patent Document 7: Japanese Patent Laid-Open Publication No. Hei 9-87526    Non-Patent Document 1: “Sol-Gelhou No Ouyou” (“Application of Sol-Gel Reaction”) authored by Sumio Sakka and published by Agne Shofusha in 1997.